My thoughts of the "final upgrades" for towers and their use:
Archer Tower:
Musketeer:
Mainly nice for the massive range, highest tower range in game easily. However the abilities aren't that great. Sniper shot is straight up worse than arcane wizard's death ray (2 second shorter cooldown, but not a guaranteed kill) Shrapnel is decent, but unreliable due to the fact that placement of musketeer's isn't gonna be in the best AOE zones
Rangers:
I prefer these vastly to the Musketeers. Their DPS is considerably higher due to the massive rate of fire and poison. The traps are very useful as they not only deal a lot of damage but they trap enemies for up to 3 seconds.
Barracks:
Barbarians:
Only good for the throwing axe in my opinion. Traps don't really do much since the point of barracks is to slow people down in general. Double axes is decent but once you get to the later levels all the demons that explode on death kill barbarians that get in too close. Throwing axe is good for additional anti-air firepower and if you have a meatshield in front of you to spam the throwing axes with impunity.
Paladins:
I prefer these in general because the shield upgrade and the heal upgrade makes them **** hard to kill. Heavy armor is ridiculous first of all. It's annoying as hell having to use non magic towers on heavy armor opponents, so just imagine all the enemy minions having to attack your paladins with high armor. Also, the holy strike gives them good damage that's AOE so it is effective against multitudes of enemies.
Mages
Sorcerer Mage:
Good mostly for the golem. Golem is like a full upgraded group of paladins, okay, slight exaggeration, but really, tons of health and AOE damage, and it still spawns every 8 seconds. THe polymorph just sucks compared to death ray, same cooldown, still only affects a single target. Instead of outright killing the target, it turns them into a sheep.
Arcane Wizard:
I like to place these near the end of the path. Since my other towers likely have killed all the fodder enemies, death ray is only gonna be targeting the strong ones that get through. Teleport also makes anybody else go through your gauntlet of towers once again.
Artillery
Big Bertha:
Not a fan of the Dragonbreath Launcher missles. However the cluster bomb has amazing AOE and clears out skeletons and other mobbed up fodder enemies like nothing else.
Tesla:
I like the AOE around the tower with every shot, which makes up for the target limit (of 5) from the electric bolts. Used at a 180 degree corner this thing is godly because of the AOE damage around the tower, but anywhere else the AOE is lacking.
TL;DR: Rangers>Musketeers, Paladins>Barbarians, Sorcerers near begining of path, and where you need golems to provide "resistance", arcane wizards at the end of paths. Tesla at a very sharp corner in the path, otherwise Big Bertha. That's all my opinion though.